The present invention relates to a radio communication system including a base station and a large number of sub-stations (e.g. tens of thousands to hundreds of thousands of sub-stations) and a radio communication apparatus used in the radio communication system.
Heretofore, there have been communication systems in which data transmitted by radio from a large number of sub-stations is received by a base station to collect data from the sub-stations. As an example thereof, there is known a telemeter system or a telecontrol system which is used for questionnairing TV viewers, for entertainment (for example, participation in a program), for TV shopping, for ensuring the security, for monitoring the safety by gas or electricity supply enterprises, for measurement of the consumption known gas, electricity or water or the like. In such a communication system, when transmission signals are transmitted from a plurality of sub-stations to a base station at the same time, the base station cannot identify the transmission signals from the sub-stations. Accordingly, the transmission from the sub-stations is controlled in accordance with a predetermined data transmission procedure so that the plurality of sub-stations do not transmit the signals at the same time.
In the case of the HDLC (High-level Data Link Control) procedure which is a representative example of the data predetermined transmission procedure, a flag, an address, a control signal and the like constituted by several tens of bits in total are added to an information field constituted by data of, for example, about 256 bits to 2048 bits. In the telemeter system or the telecontrol system as described above, in addition to the above additional information, a bit synchronizing signal, a frame synchronizing signal, an error detection code and the like constituted by several tens of bits are further added.
Furthermore, in the above communication system, bidirectional communication is performed by means of the frequency multiple or time division multiple method. It is necessary to be able to transmit data in one direction (for example, from the sub-station to the base station) whether transmission conditions of data in the other direction (for example, the base station to the sub-station) is good or bad. As confirmation methods of transmission of data, there are known polling methods in which the base station confirms the presence of a transmission request in the sub-stations, a method using the CSMA/CD (Carrier Sense Multiple Access/Collision Detect) method to transmit a request of line connection or information from the sub-station, a method of controlling transmission of data in the sub-station in response to the ACKINAK signal indicating whether the base station can receive transmission data or not, a method of successively shifting the transmission right of data among the sub-stations, and the like.
Any of these methods requires the additional identification information (ID) of an originating station or the addition of an error correction code to data to be transmitted, or the provision of sufficient space (guard time) between data to be transmitted by the sub-stations in the burst manner, or the transmission and receiving of ACK/NAK signal or, the transmission and receiving of a signal indicating shift of the transmission right. In the above communication system, generally, because of the fact that the number of the sub-stations is not so large and data transmitted by the sub-station is about several tens of bits or more, even if the bit synchronizing signal, the frame synchronizing signal, the error correction signal and the ACK/NAK signal are transmitted and received or the guard time is sufficiently ensured, the throughput of data transmission is not degraded greatly.
However, in the method in which the transmission data to which the additional information of several tens of bits as described above is added is transmitted, when data having an extremely short bit length of about two or several bits are transmitted by numerous (for example, several tens of thousands or hundreds of thousands) sub-stations, the length of the additional information added to the transmission data is lengthened as compared with the transmission data and thereby degrades the throughput remarkably. Consequently, in the above conventional method, it is difficult to transmit data by hundreds of thousands of sub-stations in an extremely short time (for example, about several seconds) and receive the data by the base station.
On the other hand, in typical radio communications, the intensity and the phase (incoming phase) of the transmission signal which has reached the receiving station (base station) are vatied due to variation (fading) of loss in propagation of radio waves. When an amount of variation in the intensity and the phase of the transmission signal which has reached the base station exceeds a permissible amount of the receiving system of the base station, data between adjacent time slots interfere with each other and it is difficult to separate the data. The fading due to variation of weather conditions is generally slow in the VHF band and a lower area of the UHF band. However, the variation speed of the fading produced by movement of a reflective object such as airplanes at a long distance is relatively fast. Such a fading having the fast variation speed does not matter when the directivity of an antenna is sharp. However, when the numerous sub-stations are distributed spatially, the so-called long delay distortion that direct waves and reflection waves reflected by a reflective object at a long distance interfere with each other occurs so that reception of data is made difficult since the directivity of the antennas is not so sharp.
In mobile communication systems such as an automobile telephone system and a portable telephone system, since the receiving intensity of the transmission signal is varied depending on a distance between the sub-stations and the base station, the fading and the like, transmission power of the sub-stations is controlled. There is one case where the control of the transmission power of the sub-stations is made by the base station and another case where the control is made by mobile terminals. However, in either case, since the fading speed due to change of a traveling speed of an automobile is very fast and large errors occur (due to either a time difference from detection of the intensity of a received signal to actual control of transmission power or, a difference between a received frequency and a transmission frequency) the transmission power is controlled on the basis of an average received level of the received signal. Accordingly, the transmission power is not controlled finely in such transmission power control schemes. For example, a general control step of the transmission power is coarse. For example, (i.e,. about 4 to 10 dB).
In the communication system in which the transmission signals from the numerous sub-stations are concentrated temporally, such large control steps make it difficult to uniformly control intensity in the case where the transmission signals transmitted from the sub-stations by the above control methods reach the base station, and still avoid the interference between the transmission signals from the sub-stations adjacent to each other in the time relation so that the throughput of data transmission can be improved.
It is an object of the present invention to provide a radio communication system in which a base station can collect data of the short bit number from numerous sub-stations in an extremely short time.